Thermal management is a major concern in the design of electronic components. All aspects of electronic system thermal management and the role of thermally conductive joining materials are discussed in ASM International's Electronic Materials Handbook, Volume 1, Packaging, 1989. Heat generated during equipment operation must be removed in order to avoid circuit damaging temperature buildups. The failure rate of semiconductor devices (chips or dies) increases exponentially with increasing temperature due to irreversible degradation of the transistor junctions. A major pathway for heat removal in electronic assemblies is by conductive diffusion of the heat through thermally conductive materials.
On-going electronic design trends dictate the need for improved thermal management materials. Improved electronic performance is accomplished by circuit miniaturization, closer component spacing, and by increasing power levels to increase circuit speed. These changes result in a higher heat flux that must be removed. The present state of the art needs materials with improved thermal transfer properties to improve removal of heat from components thereby leading to increased electronic equipment reliability and service life.
Thermally conductive joints transfer heat in electronic assemblies between physically connected parts. They are an important part of most heat removal paths. The heat transfer efficiency of these joints is defined by the concept of thermal resistance: EQU R=t/kA, (1)
where R is thermal resistance (.degree. K/watt), t is the thickness of the joint (m), k is thermal conductivity (W/m.multidot..degree. K), and A is the area of the joint (m.sup.2).
The lower a joint's thermal resistance (R) is, the greater is its heat transfer efficiency. This results in a lower temperature rise for a device at a given power level. As illustrated by equation (1), decreasing the thickness (t) of a joint decreases its thermal resistance (R). For materials that have the same thermal conductivity in all directions (thermally isotropic), this also makes thickness uniformity desirable. Otherwise, non-uniform heat flow rates will occur between the thick and thin portions. Increasing the thermal conductivity of the joint material and/or the area A decreases the thermal resistance (R). This makes it a requirement that any joint material conform to all the available surface area and not leave gaps or voids.
There are three types of thermally conductive joints important to electronic equipment:
(1) A bare contact between two rigid materials is the simplest joint. This joint cannot hold the materials together on its own and must have support provided from some other source. The thermal efficiency of this joint is related to how close the joint surfaces fit together. On a microscopic scale, the materials will only make point contacts leaving air gaps covering most of the contact area. Due to the extremely poor thermal conductivity of air (0.035 W/m.multidot..degree. K), the thickness of this gap must be reduced as much as possible. This thickness is determined by the smoothness and precision of the fit between the contact surfaces. Costly machining operations are typically required to allow these joints to transfer heat efficiently. Bare contact joints are also subject to corrosion and contamination problems.
(2) The bonded joint is the most common type used in electronic assemblies. A thermally conductive adhesive joining material adheres to the joint surfaces to hold the surfaces together and conducts thermal energy between the joint surfaces. Usually a thermally conductive adhesive flows over a joint surfaces when the joint is formed. The thermal efficiency of this joint is determined by how completely the adhesive covers the joint surface, the thermal conductivity of the adhesive layer and the thickness of this layer.
(3) The gasketed joint is increasingly being used in electronic assemblies. The gasket is a thermally conductive solid sheet of joining material that does not flow when the joint is formed. The gasket joint's thermal efficiency is determined by how closely the gasket joining material conforms to the joint surface, the thermal conductivity of the gasket joining material and its compressed thickness. The gasket joining material is typically a rubber (elastomeric) sheet. A joint with this material in it is typically held together with clamps or by pressure sensitive adhesive on the surface of the rubber. In some cases the rubber may have pressure sensitive adhesive properties of its own.
A desired characteristic of all thermally conductive joining materials is that they be able to intimately contact the joint surface by conforming to its shape. The joining material does this during the fabrication process by flowing as a liquid or by compressing as a solid. This allows surfaces that are not perfectly matched to be efficiently, thermally joined. This typically eliminates the surface machining required for bare contact joints and leads to lower manufacturing costs.
Another desired characteristic of thermally conductive joining materials is a low thermal joint processing temperature, which minimizes problems caused by coefficient of thermal expansion (CTE) stress. Die bonding usually entails heating the joining material for bonding with the surface to be joined. Normally the semiconductor has a CTE that is different from the substrate it is being bonded to. Therefore, the greater the temperature excursion during the processing of the die bond, the greater the CTE stress on the semiconductor chip when it cools down. This can lead to damage or lower reliability for the device. Therefore a lower processing temperature is desirable for the die bonding material.
CTE stresses can also be decreased by lowering the in-plane stiffness of the die bond itself. Therefore a die bond material with a lower in-plane rigidness (modulus) is desirable. Higher bond material thickness lowers the CTE stresses but this normally increases thermal resistance. In order to minimize CTE stress, a particularly desirable combination of attributes for a die bonding material would be a low processing temperature, a low in-plane modulus, and higher thermal conductivity to allow increased bond material thickness with the same or lower thermal resistance.
Often the electrical properties of a thermal joining material are important. The thermal joint for some electrical designs is either electrically conductive or insulating. It is desirable therefore that an improved thermal joint be capable of being either electrically conductive or non-conductive.
The most common forms of materials used for electronic joints are pastes and films. Pastes are liquid materials that are typically applied by hand application or by a machine controlled syringe. Films are thin, controlled thickness sheets of the joining material. Films can be either semi-liquid or solid materials that become liquid during processing or rubber materials that will conform under compression. Films offer advantages in uniform thickness control, reduced voids and less material waste.
The most commonly used materials for thermally conductive joints are solders, silver-glass eutectic alloys, and organic polymers. All of these materials when used in joints have serious deficiencies. For example, a major problem with thermally conducting solder joining materials is that higher thermal conductivity is attainable only by using undesirable high processing temperatures. The thermal conductivity of solders range from 35 to 73 W/m .degree. K. Solder thermal joints must also have a melting point that significantly exceeds subsequent processing temperatures. This limits processing temperatures with solder to a relatively high range, which leads to CTE stress problems. Solder is also subject to fatigue cracking caused by CTE stresses generated by temperature cycling during normal equipment operation.
Silver-glass bonds are normally achieved by applying a mixture of silver flake loaded glass that is dispersed in an organic matrix and firing it at 320 to 460.degree. C. The organic matrix is burned out and an eutectic alloy bond material is formed. The high temperature firing leads to CTE stress problems, requires extra coating steps, and can have oxide formation problems. Silver-glass has a thermal conductivity of 40 to 75 W/m.multidot..degree. K.
Organic polymers typically have a low processing temperature, low manufacturing costs, and low in-plane modulus. Organic polymers also exhibit excellent compressibility, thus intimately contacting the joint surfaces being mated even though these surface do not exactly match in shape. However, organic polymers also have very low thermal conductivity (0.1 to 0.3 W/m.multidot..degree. K).
Thermally conductive fillers have been added to organic polymers to increase the thermal conductivity. While the fillers increase the thermal conductivity of organic polymers significantly, the thermal conductivity achieved is still only a tiny fraction of the conductivity of the fillers themselves. Silver has a thermal conductivity of 420 W/m.multidot..degree. K, but silver filled polymers achieve only 2 to 6 W/m.multidot..degree. K. Diamond filler has a thermal conductivity of over 1500 W/m.multidot..degree. K, but diamond filled polymers achieve only 8 to 11.5 W/m.multidot..degree. K.
Thermally conductive fibers have also been used to fill polymers to improve thermal conductivity. For example, Eddy et al., in U.S. Pat. No. 4,321,033 (1982) describes carbon or metal fibers in a brush configuration that is impregnated with an elastomer material. An improvement in thermal conductivity over silicon rubber of about 3 times is described. However, the Eddy et al composite material is not useful or adaptable as a thermally conductive joining film, because it is too thick. It is stated that the brush fabric can not be conveniently made below 30-50 mils (0.76-1.27 mm) in thickness. Yet, it is desirous that a thin film of joining material be less than 30 mils.
Lee et al., in U.S. Pat. No. 4,729,166 (1988), describe a means for fabricating an anisotropic electrical conductor having conductive fibers that run through the thickness so that they extend from surface to surface. These fibers are also oriented to extend in a direction that is substantially perpendicular to the surfaces of the conductor. The composite material in Lee et al., however, lacks the proper compressibility required to be an effective thermally conductive joining film. While polymer based matrix materials may exhibit sufficient local compressibility, the fibers extending between the matrix surfaces act as small rigid columns that resist compressive loads and therefore are incapable of compressing locally to accommodate a variable gap between substrates. Without the ability to compress, these composite materials can efficiently thermally join only two perfectly matched surfaces. Even with two matched surfaces, the fibers would have to be exactly the same length and would have to exactly match each top and bottom surface.
In order to locally compress the prior art films, the fibers therein must start to buckle under the compressive load before the elastomer around them can start to compress. A simple column buckling analysis using Euler's formula is described in Standard Handbook of Machine Design, chapter 15, 1986, McGraw-Hill, Inc. The unrestrained load (P.sub.CR) on the ends of a fiber required to start it to buckle is given by: EQU P.sub.CR =.pi..sup.L EI/L.sup.2, (2)
For a round fiber the moment of inertia, l=0.049d.sup.4, therefore: EQU P.sub.CR =0.049.pi..sup.2 Ed.sup.4 /L.sup.2 (3)
In this case a conversion to the force (F.sub.cr) applied is more useful: EQU F.sub.cr =P.sub.cr /A (4)
For a round fiber, the area A equals .pi.(0.5 d).sup.2, therefore: EQU F.sub.cr =0.1 9.pi.Ed.sup.2 /L.sup.2, (5)
where F.sub.cr is the applied critical compressive force (MPa), E is elastic modulus (MPa), d is fiber diameter (cm), and L is the length of the fiber (cm). The load required to start compressive deflection of the overall sheet is given by: EQU F=F.sub.cr V.sub.f, (6)
where F is the compressive force (MPa) and V.sub.f is the fiber volume fraction.
According to equation 5, a copper fiber of 0.01 cm (0.004 in) diameter (d), a length (L) of 0.1 cm (0.039 in) and an elastic modulus (E) of 131,000 MPa (19 psi) would require a force (F.sub.cr) of over 807 MPa (116,993 psi) to start the fiber elastically buckling, thus allowing the sheet to compress. Even assuming a low fiber volume (V.sub.f) of 0.10, a force (F) of 80.7 MPa (11,699 psi) would be required to compress this material. This is beyond anything reasonable for an elastic sheet.
For a mesophase pitch based carbon fiber with a diameter (d) of 0.001 cm (0.0004 in) and a modulus (E) of 837,000 MPa (120 psi), a force (F.sub.cr) of 50.9 MPa (7,389 psi) would be required to start to buckle a fiber length of 0.1 cm (0.039 in). Being a very brittle fiber, it would break after a small buckling deformation, permanently disrupting the heat flow path. A fiber volume (V.sub.f) of only 0.10 would require 5.1 MPa (739 psi) to start deforming the sheet.
An elastic material proper for thermal joining should yield easily to finger tip pressure. Therefore, it is clear that fibers extending directly between the film surfaces where they are substantially perpendicular to the film surfaces, as shown in the prior art, are not easily compressible and thus unacceptable for an effective thermal joining film. This compressibility problem is independent of the properties of the surrounding matrix and holds true even if a liquid state could somehow be embodied for the elastomer.
In summary, there is a need for a thermally conductive joining film that has the high thermal conductivity evidenced by extending thermally conductive fibers completely through its thickness, but which is capable of local compression with minimal force in order to form an effective thermally conductive joint. There is also a need for this film to have a uniform thermal conductivity even in areas that are locally compressed to a smaller film thickness. Lastly, there is a need for an efficient, low cost process to make large quantities of high quality thermally conductive thin joining films that have superior local compression characteristics. Such a process should make the film in long roll lengths with a tightly controlled thickness.